Because of increasingly restrictive environmental regulations, there is a need to lower pollutant emissions for a variety of combustion applications. For machinery that uses steady flow combustion of either gaseous or relatively volatile liquid fuels with an overall excess of oxidant, lean pre-mixed combustion is a potential viable approach. Lean pre-mixed combustion tends to produce relatively low nitric oxide emissions because of its reduced flame temperature. Process parameters for lean pre-mixed methods of combustion involve vaporizing the fuel, if it is a liquid, mixing the fuel with air in excess of the air required to theoretically completely oxidize the fuel, and igniting and stabilizing the lean pre-mixed burning process. In practical terms, these steps are oftentimes not ideally realized because the hardware may not function as desired, may function only with substantial monitoring, control and complexity, or accomplish those steps quite imperfectly, producing relatively higher than theoretically achievable pollutant emissions. Accordingly, in terms of emission levels, simplicity and reliability, there are degrees of how well a particular lean pre-mixed design works.
A substantial number of lean pre-mixed combustion designs have been proposed in the past. Often, such designs include combustion systems which use gaseous fuel injection, liquid spray injection, swirling air flow, pre-mixing zones, lower flow speed, and flame stabilization regions. To the present, however, it has been a problem to combine liquid and gaseous fuel in a combustion process and apparatus to reliably achieve lean pre-mixed combustion.